Thin films deposited from solution by high throughput processes (>1 mm/s) generally have small crystalline grain size and, therefore, are adversely affected by the properties of grain boundaries. As a result, such films typically have lower electronic carrier mobility and larger charge trap densities compared to films with large crystalline grain size, and are thus less useful for many electronic applications.
In prior art, the dominant method of increasing the crystallinity (e.g., increasing the crystalline grain size) of solution-processed thin films is to reduce solvent evaporation by enclosing the surface during or after the solution deposition in order to reduce the rate of solvent evaporation and thus slow down the drying process. However, this process is difficult to scale to a high throughput process.
Additionally, the deposition of multiple thin film layers from solution cannot be easily accomplished by previous solution processing methods because each solution layer damages or completely dissolves the layer beneath. Solution-based methods used in the production of, for example, organic electronics, organic photovoltaics, and organic light emitting diodes are thus limited in how many layers can be effectively stacked. Typical processes include drop casting, spin processing, blade coating, slot-die coating, and various printing methods. The current state of the art for solution-processed films is to deposit only a single soluble layer. In the case of organic photovoltaics, two materials are mixed within such a single layer, and natural phase separation causes the materials to separate into a complex nanomorphology, known as a bulk heterojunction (“BHJ”). However, the nanomorphology obtainable by natural phase separation is known to be far from optimal. In other markets, where nearly planar interfaces are preferred and/or where materials do not readily phase separate with the correct nanomorphology (such as, for example, organic transistors, light emitting diodes), deposition methods resulting in BHJs are not appropriate.
Other approaches to solving this problem have previously been developed by making use of the different solubilities of specific materials, i.e., cases where solvent #1 is used to deposit layer #1 and solvent #2 for layer #2, and the material of layer #1 is chosen such that solvent #2 does not significantly dissolve or damage it. In some cases, the material of layer #1 is engineered in such a way as to change from a soluble material to a material that is insoluble, for example, to polymerize during a heating step. These methods depend on specific choices of the materials and solvents, and demand that material #1 have properties that are fairly novel.
Therefore, based on at least the foregoing, there is a need for solution-based high-throughput deposition methods which yield thin films having large crystalline grain size and/or multiple film layers.